A class of antihypertensive agents such as prazosin, disclosed in U.S. Pat. No. 3,511,836, which is a 2-substituted quinazoline derivative containing a quinazoline ring system, has been proven effective in the clinic acting as a .alpha..sub.1 -adrenoceptor antagonist. While 3-substituted quinazolinones such as ketanserin, thioketanserin disclosed in U.S. Pat. No. 4,335,127, and SGB-1534 disclosed in Japan J. Pharmacol. 1987, 44, 35 have been found to have antihypertensive activities by a serotonin-S.sub.2 and .alpha..sub.1 -adrenoceptor antagonist, respectively. During the course of our synthetic studies on the fused quinazoline ring system, we have synthesized the angularly tricyclic condensed quinazoline derivatives such as 2-substituted methyl-2,3-dihydroimidazo[1,2-c]quinazoline derivatives which would possess a rigid structural feature necessary to elicit the biological activities of both ketanserin and SGB-1534 and have been shown potent lowering blood pressure acting as a .alpha..sub.1 -adrenoceptor antagonist (U.S. patent application Ser. No. 07/744,534, filed by Ji-Wang Chern, et al. and is now U.S. Pat. No. 5,158,953. Recently, it was reported in Drug of the Future , 1989, 14, 400 and Brit. J. Pharmacol. 1988, 93, 702-14 that .alpha..sub.1 -adrenoceptor antagonists can be used for the treatment of dysuria which is due to the prostatauxe.
Nevertheless, more potent and clinically effective antihypertensive agents are still needed.